A lithium ion battery, which is a kind of nonaqueous battery, has been used widely as a power source of portable equipment such as a mobile phone and a notebook-sized personal computer, because of its high energy-density characteristic. With improvement in the performance of portable equipment, the capacity of the lithium ion battery tends to increase further, and thus securing safety has become important.
In a conventional lithium ion battery, a polyolefin-based porous film having a thickness of about 20 to 30 μm, for example, is used as a separator to be interposed between a positive electrode and a negative electrode. As the material for the separator, polyethylene having a low melting point sometimes is used for securing a so-called shutdown effect, namely, melting a resin forming the separator at or below a thermal runaway temperature of the battery so as to close the pores, thereby increasing the internal resistance of the battery and improving the safety of the battery at the time of short circuit or the like.
As the above-described separator, for example, a uniaxially-stretched or biaxially-stretched film is used in order to increase porosity and improve strength. Since such a separator is provided as a stand-alone film, a certain strength is required for the separator in view of workability or the like and secured by the above-mentioned stretching. However, since the crystallinity of the stretched film has increased, and the shutdown temperature has been raised to temperatures close to the thermal runaway temperature of the battery, the margin for securing the safety of the battery cannot be provided sufficiently.
Moreover, there occurs distortion in the film due to the stretching. Thus, when the film is exposed to high temperatures, shrinkage will occur due to residual stress. The shrinking temperature is very close to the melting point, that is, the shutdown temperature. As a result, in the case of using a polyolefin-based porous film separator, when the temperature of the battery reaches the shutdown temperature during anomalies in charging or the like, the electric current must be decreased immediately for preventing increase of the battery temperature. If the pores of the separator are not closed sufficiently and the electric current cannot be decreased immediately, the battery temperature will rise easily to the shrinking temperature of the separator, causing a risk of heat generation due to internal short circuit.
In order to prevent such a short circuit caused by the thermal shrinkage, a microporous film using a heat-resistant resin or a nonwoven fabric as a separator have been proposed. For example, Patent document 1 discloses a separator using a microporous film of wholly aromatic polyamide, and Patent document 2 discloses a separator using a polyimide porous film. Further, Patent document 3 discloses a separator using a polyamide nonwoven fabric, Patent document 4 discloses a separator including a base of a nonwoven fabric using aramid fibers, Patent document 5 discloses a separator using a polypropylene (PP) nonwoven fabric, and Patent document 6 discloses a technology regarding a separator using a polyester nonwoven fabric.
However, although the microporous films using the heat-resistant resins such as polyamide and polyimide have an excellent dimensional stability at high temperatures and can be made thinner, they are expensive. Also, the nonwoven fabrics using the heat-resistant fibers such as polyamide fibers and aramid fibers have an excellent dimensional stability but are expensive. The nonwoven fabrics using PP fibers or polyester fibers are inexpensive and excellent in dimensional stability at high temperatures. However, since the pore diameter is too large in the state of a nonwoven fabric, these nonwoven fabrics having a thickness of equal to or smaller than 30 μm, for example, cannot prevent the short circuit due to contact between the positive and negative electrodes or the short circuit due to the generation of lithium dendrites in a sufficient manner.
Furthermore, a technology has been proposed in which a nonwoven fabric or the like made of a low-cost material is used as a separator by various processings. For example, Patent document 7 discloses a separator obtained by filling a heat-resistant polybutylene terephthalate nonwoven fabric with polyethylene particles, alumina particles, etc., and Patent document 8 discloses a separator obtained by layering a heat-resistant separator layer formed principally of inorganic particles and a thermal-melting separator layer formed principally of organic particles such as polyethylene particles.
Patent document 1: JP 5(1993)-335005 A
Patent document 2: JP 2000-306568 A
Patent document 3: JP 9(1997)-259856 A
Patent document 4: JP 11(1999)-40130 A
Patent document 5: JP 2001-291503 A
Patent document 6: JP 2003-123728 A
Patent document 7: WO 2006/62153 A
Patent document 8: WO 2007/66768 A
Now, lithium reacts vigorously with water. Therefore, in order to secure excellent properties and reliability of the lithium secondary battery, it is important to remove water from the battery as much as possible. Also, in the lithium secondary battery, when a slight amount of water is mixed inevitably in an organic electrolyte solution or water is adsorbed in other electrode materials, the reaction represented by the formula below occurs, thus generating a halogen acid such as hydrogen fluoride (HF).2LiPF6+12H2O→12HF+2LiP(OH)6 
The hydrogen fluoride generated in the above reaction causes a problem of deteriorating materials constituting the battery such as a current collector and a positive active material and further degrading the battery performance. In such a battery whose constituent materials have been deteriorated, the internal resistance increases. Additionally, an aluminum foil is generally used as a positive current collector. When the positive current collector is corroded by hydrogen fluoride, eluted metal ions are deposited on the negative electrode, thus causing the deterioration of self-discharge characteristics. Moreover, water entrained into the battery sometimes generates not only the halogen acid such as hydrogen fluoride but also hydrogen. The hydrogen fluoride and hydrogen may cause battery swelling or a decrease in the charge-discharge cycle characteristics of the battery when the battery is stored in a high-temperature environment.
In the field of lithium secondary batteries, the recent dehydration technology is improved to achieve about 20 ppm or lower water content of an organic solvent. Further, the amounts of water contained in the positive electrode, the negative electrode and the separator also can be reduced by a decompression treatment down to 200 ppm or lower in terms of water content of the organic electrolyte solution when it is assumed that the water has moved into the organic electrolyte solution in the battery. For example, in a polyolefin-based porous film, which is a conventionally known separator, it is possible to remove water relatively easily by a decompression heat treatment at 100° C. or lower, thereby achieving the above-noted water content.
However, in the above-described conventional separator containing the inorganic particles and the organic particles, there has been a possibility of entraining unnecessary water in the battery unless the amount of water in the separator including the water contents of particles to be used and a binder used for adhesion is controlled strictly.